The present invention relates to memory devices and, in particular, to cross-point diode memory devices in which an anisotropic semiconductor sheet is employed as a two-dimensional array of fuse-and-diode memory elements.
As computer processors and digital data storage devices have become more and more commonly used in consumer electronics, the need for high-capacity, but inexpensive, digital storage devices has greatly increased. In some cases, the lack of sufficiently inexpensive, high-capacity digital memory devices has inhibited marketing of new consumer electronics devices that store large amounts of digital data during operation. An example of consumer electronics devices that need inexpensive, high-capacity digital memory is high-resolution digital cameras. Although increasing in popularity, digital cameras are currently still too expensive for wide popular acceptability. Moreover, digital cameras can be manufactured with much greater resolution, but the digital data storage requirements for the higher-resolution images captured by these higher-resolution digital cameras further increase their operating cost.
Digital data is commonly stored on rotating magnetic disk drives and in semiconductor-based memories, such as EEPROMS and flash memories. Disk drives are expensive, consume rather large amounts of power, and are insufficiently robust for many consumer devices. Flash memories are more robust but, because they are produced by the photolithographic techniques used to produce microprocessors and other semiconductor electronic devices, they are currently too expensive for use in inexpensive consumer electronic devices, or for write-once consumer applications, such as storing digital images captured with digital cameras.
Recently, a new cross-point diode memory has been developed to serve as a high-capacity, write-once memory in consumer electronics devices, such as digital cameras. FIG. 1 is a cut-away isometric view of a portion of a cross-point diode memory module. The cross-point diode memory module comprises a number of identical, stacked layers. Layers 101-113 are shown in FIG. 1. Each layer comprises a substrate 116 on which a two-dimensional memory array 118 is formed. The two-dimensional memory array comprises row and column conductive elements, or lines, that together create a grid-like pattern. The row lines of the two-dimensional memory array are electronically coupled to input/output (xe2x80x9cI/Oxe2x80x9d) leads 120-123 via a row multiplexer/demultiplexer circuit 124. The column lines are coupled to column I/O leads 126-129 via a column multiplexer/demultiplexer circuit 130. The row I/O leads 120-123 and the column I/O leads 126-129 are electronically connected with contact elements, such as contact element 132 to which row I/O lead 120 is connected, that extend along the sides of the cross-point diode memory module to electronically interconnect the row I/O leads and column I/O leads of all layers 101-113 of the memory module. Each grid point intersection of a row line and column line in the two-dimensional memory array 118 represents a single binary storage element. Note that, as will be discussed below, the row lines do not physically contact column lines at grid-point intersections, but are coupled through a memory element. Each memory element can be electronically accessed for reading or writing by producing appropriate electronic currents in the contact elements, such as contact element 132.
FIG. 2 illustrates a single memory element of a two-dimensional memory array from a layer of a cross-point diode memory device. In FIG. 2, a portion of a row line 202 is shown orthogonal to, and above, a portion of a column line 204. As discussed above, the intersection of the row line 202 and column line 204 corresponds to a single bit of stored digital information. In a cross-point diode memory, intersecting row and column lines, such as row 202 and column 204 in FIG. 2, are electrically coupled through a memory element 206. In electrical terms, the memory element comprises a fuse 208 and a diode 210 in series.
Digital binary digits, or bits, can have one of two possible values, xe2x80x9c0xe2x80x9d and xe2x80x9c1.xe2x80x9d Physical media that store digital data in digital memory devices generally have two different physical states that can be interconverted and that can be detected via a physical signal. In the case of a cross-point diode memory element, such as memory element 206 in FIG. 2, one of the two binary states is represented by an intact fuse 208, and the other of the two binary states is represented by a blown fuse 208. Unlike a read/write memory, such as a hard disk drive, a cross-point memory element can be converted from the fuse-intact state to the fuse-blown state only once, and hence cross-point diode memories are generally write-once memories. The diode 210 component of the memory element 206 serves to eliminate undesirable electrical paths between row and column lines. When the fuse component 208 of a memory element 206 is intact, the electrical resistance of the memory element 206 is relatively low, and currents can pass between the row line 202 and column line 204. In order to change the state of the memory element from the fuse-intact state to the fuse-blown state, a much higher current is passed through the memory element 206 between the row line 202 and the column line 204, resulting in electrical failure of the fuse component 208. Once the fuse component 208 has failed, the electrical resistance of the memory element 206 is relatively high, and comparatively little or no current can pass from the row line 202 through memory element 206 to the column line 204. Thus, a memory element of the cross-point diode memory can be written, or changed from the fuse-intact state to fuse-blown state via a high current signal, and the state of the memory element can be determined by determining whether the memory element passes a comparatively low current signal.
The cross-point diode memory module illustrated in FIGS. 1 and 2 may serve as a high-capacity, but inexpensive digital data storage component of consumer electronics devices provided that an inexpensive and efficient technique can be found for manufacturing the fuse-and-diode memory elements, such as fuse-and-diode memory element 206 illustrated in FIG. 2. Thus, designers and manufacturers, consumer electronic devices requiring inexpensive, high-capacity digital data storage components have recognized the need for a inexpensive and efficient method for manufacturing cross-point diode memory elements.
One embodiment of the present invention provides a thin sheet of anisotropic semiconductor material that can be sandwiched between row and column lines of a two-dimensional memory array of a cross-point diode memory layer. The anisotropic semiconductor material is composed of small-molecule organic compounds that can be formed in stable films, one on top of the other, or laminated together, to produce a donor/acceptor-organic-junction device. A donor/acceptor-organic-junction device is, by its nature, a diode. The films can be manufactured to have relatively low electrical resistivity in a direction perpendicular to the plane of the films and to have relatively high electrical resistivity in the plane of the film, and are thus anisotropic. Because the semiconductor sheet is anisitropic with respect to electrical resistivity, the memory elements do not need to be manufactured by expensive photolithographic techniques or otherwise manufactured to correspond to the row line and column line dimensions and orientations, but instead arise in the anisotropic donor/acceptor-organic-junction material via proximity to memory-array grid points.
The anisotropic semiconductor sheet passes current in one direction between intersecting row and column lines of a two-dimensional memory array. When a high-voltage or high-current signal passes between a column line and a row line, the small-molecule compounds forming the anisotropic seiconductor sheet vaporize, leaving a gap in the anisotropic semiconductor sheet at the intersection of the row line and column line through which the high-current or high-voltage signal is passed. Once a gap has been formed, a relatively low-current signal can no longer pass between the column line and row line, and hence the anisotropic semiconductor sheet serves as the fuse component of a cross-point diode memory element. The anisotropic semicondcutor sheet composed of films of small-molecule organic compounds thus serve as an array of fuse-and-diode memory element at each grid point of the two-dimensional memory array of a cross-point diode memory device.